The epidemiology of multiple sclerosis (MS) points to an environ- mental factor. One hypothesis for MS is that the disease is initiated by a virus which persists in the central nervous system (CNS), and that subsequent attacks are the result of an immune- mediated response. The chronic demyelinating disease of mice caused by Theiler's virus (TV) is a laboratory model condition in which the DA strain produces a biphasic disease; there is initial acute neuronal infection followed by chronic demyelinating disease with persistence of virus in the CNS. Experimental literature to date seems to establish that demyelination occurs as a result of 3 major factors: (a) virus-must persist for a prolonged time (b) virus may directly kill oligodendrocytes and destroy myelin without the need for an immune response (c) the immune response may destroy myelin. At present, it is generally agreed that viral persistence is necessary for demyelination to occur, but there is some debate as to the importance of the other 2 factors. The mechanism that allows virus to persist within the CNS is currently unknown. We propose studies of primary cultures of mouse brain cells and cerebellar explants to study viral infection and persistence, in- vitro demyelination, and the importance of macrophages in TV demyelinating disease. Infection of primary brain cell cultures with the DA strain of TV results in a persistent infection with little visible cytopathic effect. Host cell mechanisms, particularly proteolytic mechanisms of macrophages which may lead to persistent infections rather than lytic infections, will be investigated. We will use an in-vitro model to study the role of macrophages in persistent infections, and to ascertain if host genetic factors determine the capacity of a cell to maintain a persistent infection. ln addition we will study in-vitro demyelination of infected cerebellar explants, and we will also use an in-vivo assay to examine targets of the immune effector cells.